The resolution of imaging systems (e.g., telescopes) is typically proportional to the aperture size (also referred to as light collecting area) of the systems. Conventionally, a larger aperture size can be achieved by using lenses or mirrors of a larger diameter. For example, the Hubble Space Telescope has a single aperture (filled with monolithic lenses) with a diameter on the order of about 2.4 meters. A telescope of this size almost completely fills a space shuttle cargo bay. Therefore, the aperture diameter of the Hubble Space Telescope is already the maximal diameter a space-based telescope can practically have. Accordingly, the resolution of a single aperture space imaging system is already at its practical limit.
Increasing the diameter of monolithic lenses or mirrors may introduce at least two challenges. First of all, the cost and technical difficulty to manufacture a lens increases significantly as the diameter of the lens increases due to, for example, the stringent requirements on the surface quality of the lens. In addition, lenses of larger diameter are normally heavier, thereby requiring stiff support structures. Normally, the entire telescope support structure and the lens are steered in order to aim the lens at the desired field of view. Sufficient stiffness of the rigid telescope support structure is desired such that reaction forces on the support structure during positioning do not adversely affect the sensitive image collecting optics of the telescope. Stiffness typically translates into added weight and cost, which can be primary constraint factors in space applications.
An alternative to imaging systems using monolithic lenses is to fabricate lenses (or reflectors) as a number of segmented and foldable components. This option can reduce fabrication costs and weight and can package large lenses or mirrors into the cargo bays of existing space launch vehicles. Therefore, it is conceivable to build a multi-meter diameter segmented full aperture imaging system (i.e., the aperture is filled up with lenses or mirrors), which can gather more light and have a higher resolution than that of the Hubble Space Telescope.
However, in space applications, a multi-meter diameter segmented full aperture system is compactly stowed within the cargo space of a launch vehicle in order to be sent into its orbit. Stiff foldable support structures are normally employed for the stowing. Sometimes thin deformable mirrors are used to save weight, in which case complex and potentially high bandwidth adaptive optics are also included in the imaging system for positioning the deformable mirrors. Sometimes the imaging system may also be implemented as a phased array (either on the same satellite or a separate satellite), which then typically also includes complex piston and pupil matching control. Therefore, a multi-meter diameter segmented full aperture imaging system could still be heavy and have high technical risk.
Another alternative to imaging systems using monolithic lenses is to use sparse apertures (also referred to as dilute apertures, sparse arrays, sparse aperture array, or sparse distributed apertures). In a sparse aperture, a number of sub-apertures (e.g., lenses or mirrors) are sparsely distributed within an aperture area (i.e., the aperture area is only partially filled with lenses or mirrors) to synthesize the performance of, for example, a monolithic lens (or any other imaging optics) filling the entire aperture area.